OBJECTIVE: To demonstrate that individuals with positive disposition compared to those with negative dispositions have lower levels of stress-related hormones, better immune function, and are more likely to survive stressful social and physiological insults. RESULTS Evidence for a link between negative affect and disease progression exists. However, there is almost no scientific evidence examining the effects of positive affect on disease progression. In humans, social stress is ubiquitous and often leads to psychological and physical disability. Numerous studies have demonstrated that loss of a significant other is associated with depression and anxiety, as well as increased medical illnesses and increased mortality. While the mechanisms that underlie these effects are not well understood, recent studies have demonstrated linkages between the brain and a variety of physiological systems, including the immune system. These anatomical studies suggest a pathway by which stress and emotions may regulate the immune system. Different individuals experience the same stressful life events in very different ways. However, little is known about the factors that increase or decrease an individual's vulnerability to stress and its effects on health. Individual responses to stressful events appear to be more complexly related to disease progression than was earlier thought. It appears that it is the interaction between stressful events and an individual's temperament that determines how individuals perceive and cope with stress. Failures in the ability to cope result in negative affective states that may increase one's vulnerability to certain disease processes. For example, a recent study demonstrated that the presence of depression greatly increased the risk of death in individuals who recently suffered heart attacks. To begin to understand the interaction among the factors involved in survival, researchers from the WRPRC have combined efforts and expertise with researchers at the Caribbean Primate Center in Cayo Santiago Puerto Rico to exploit the unique opportunity provided by the free ranging colony and its inherent naturalistic stressors. Affiliative behaviors have been quantified and individual differences in those measures appear to be trait-like. Among four year old males, those that were most avoidant had higher cortisol levels. In contrast, those animals that gave the most aggression had lower levels of cortisol. KEY WORDS emotions, development, defense, behavior, brain, affect, longevity, immune, stress, health, HPA axis, hormones